/**
* @author AlbertDarren
* @contact 2563491540@qq.com
*/
#include <cstdio>

using namespace std;

int main_26()
{
	int n,sum=0;
	scanf("%d",&n);
    while (n)
    {
        sum+=n--;
    }
    printf("%d\n",sum);
	return 0;
}
